Fixing bad geometry on a complex mesh
I looked over the file and found that over huge parts of the object there are faces overlapping other faces:


Some connected some not. While using a remesh modifier with a low enough voxel (roughly 0.08) size results in the geometry to be clean again:


The resulting mesh will have a throughout higher density and i do not know if you want to retopologize your dragon, as that might be the best way to reduce the density again.

I tried to use the decimate modifier and it actually resulted in similar artifacts, so retopo seems to be the best choice.

Another option going out from the version you shared, would be to cut away half of the dragon and using the mirror modifier. Then you would have to go over half the dragon and manually remove all the overlapping faces, believe me, it will be a good timekiller. Retopo seems in my humble opinion to be the better option.

Happy Blending